The invention relates to a power circuit for performing impedance conversion of a given voltage to provide an output, in particular to a power circuit for use in a liquid crystal display (LCD) apparatus which requires a multiplicity of voltage sources.
Apparatuses have been commonly used as display means for portable communication devices such as cellular phones and pagers. In an LCD apparatus, a drive circuit is used to drive a multiplicity of display elements or pixels in a given duty cycle using a multiplicity of bias voltages as shown in FIG. 1.
The LCD apparatus of FIG. 1 comprises:
a bias circuit 11 for generating a multiplicity of bias voltages by dividing a voltage between a source voltage Vdd and a ground voltage E by a multiplicity of series resistors each having a resistance of about 1 M Ohms;
a buffer circuit 12 having voltage followers 121-123 for generating outputs by impedance conversion of the respective bias voltages;
a selection circuit 13 for selectively applying the output voltages of the buffer circuit 12 to the display elements 141 of the LCD to be activated in accordance with the display data; and
a display panel 14 on which a display pattern associated with the display data is formed by the pixels 141 thus activated.
In the duty operation by the multiplicity of bias voltages, those pixels having voltages applied to the electrodes thereof in excess of a predetermined level will be turned on.
An LCD apparatus having such arrangement must be operated at a low power on the one hand in order to maximize the life of the LCD as much as possible, but on the other hand, in order to provide a good display quality, it must be operable by a large driving power to prevent deterioration of output waveforms especially for a large capacitive load.
To meet these conflicting requirements, conventional LCD apparatuses employ a power circuit formed of voltage followers in a buffer circuit as shown in FIGS. 2 and 3.
As shown in FIG. 2, connected between the source voltage Vdd and the ground voltage E are a constant current source I11 and an N channel MOSFET Q11 connected in series with each other, providing at the node therebetween an output voltage Vo. A difference amplifier CP11 is also provided in the power circuit, having a negative or inverting input terminal for receiving an input voltage Vin and a positive or non-inverting input terminal for receiving the output voltage Vo, and generating a gate voltage for the MOSFET Q11.
In the power circuit shown in FIG. 2, a constant current i1 is provided from the constant current source I11. The input voltage Vin and the output voltage Vo are compared in the difference amplifier CP11 to control switching operation of the MOSFET Q11. The output voltage Vo is controlled to balance the input voltage Vin.
In an LCD apparatus capacitive loads are driven by combinatory voltages formed of different bias voltages, which cause such output voltage Vo to fluctuate up and down. Thus, the output voltage Vo deviates off a predetermined voltage for unspecified noise sources. In what follows a noise that causes an upward shift of the output voltage Vo will be referred to as positive noise or H noise, and a noise that causes a downward shift of the output voltage Vo will be referred to as negative noise or L noise.
In the power circuit shown in FIG. 2, as the output voltage Vo is pushed up appreciably by an H noise, the MOSFET Q11 is turned on by the output voltage of the difference amplifier CP11 to lower the output voltage Vo, until the output voltage Vo balances the input voltage Vin. Thus, the ability of the circuit to lower the output voltage Vo raised by a positive noise depends on the driving power of the MOSFET Q11.
On the other hand, if the output voltage Vo is lowered by an L noise, the MOSFET Q11 is turned off by the output of the difference amplifier CP11, and a s a result, a constant current i1 is supplied from the constant current source I11, which gradually pushes up the output voltage Vo. The output level of the difference amplifier CP11 will become high to turn the MOSFET Q11 as t he output voltage Vo equals the input voltage Vin, thereby keeping the output voltage Vo at the same level of the input level Vin. Thus, the ability of the power circuit to raise lowered output voltage Vo is determined by th e magnitude of the constant current i1 from the constant current source I11.
It is noted that the MOSFET Q11 keeps the current i1 flowing to have the output voltage Vo balancing the input voltage Vin.
In this way, in order to suppress noises, especially L noises, it is necessary to make the constant current i1 sufficiently large, which opposes, however, the aforementioned requirement that the power to drive the LCD circuit should be low.
FIG. 3 illustrates a conventional circuit with an improvement to overcome such problem as discussed above in conjunction with FIG. 2, in which a P channel MOSFET Q12 and a further constant current source I12 are connected in parallel with the constant current source I11. The basic structure and function of the improved circuit are the same as those of FIG. 2.
In the arrangement shown in FIG. 3, the MOSFET Q12 is supplied at the gate thereof with a periodic control signal for turning on the MOSFET Q12 at times when noises are supposedly likely to superpose on the output voltage Vo, thereby turning on the MOSFET Q12 to provide an extra constant current i2 from the constant current source I12 superposing on the constant current i1 from the constant current source I11, which adds to the power circuit a counteractive power against L-noises.
However, in this arrangement, the MOSFET Q12 is turned on periodically, irrespective of whether a noise exists affecting the output voltage Vo or not. Hence, although anti-L noise capacity is improved a little, the improvement cannot be a fundamental solution to the drive circuit for LCD apparatus.
xe2x80x9cWe see therefore that conventional drive circuits still suffer from a contradiction in suppressing the constant current on the one hand to prolonging the life of an LCD apparatus, and enhancing the current to suppress noises, especially L noises.xe2x80x9d
In accordance with one aspect of the invention, there is provided a power circuit comprising:
a first switching element connected between an output terminal of the power circuit and a first voltage supply;
a second switching element connected between a second voltage supply and the output terminal;
a first comparator for comparing an input voltage with an output voltage at the output terminal, to turn on the first switching element if the output voltage exceeds the input voltage;
a second comparator, having an input end and an output end, for comparing the output voltage with a reference voltage, to turn on the second switching element if the output voltage becomes lower than the reference voltage; and a reference voltage circuit for changing the reference voltage depending on a voltage value at the output end.
The second switching element of the power circuit is turned on in raising the output voltage, so that the power need to run a load is significantly reduced as compared with conventional constant current type power circuits.
The second comparator exhibits hysteresis during operation. The hysteresis of the second comparator controlling the second switching element may improve noise reduction, and hence output distortions caused by the noise in the power circuit.
By controlling the first and second comparators, respectively, so as not to make the first and the second switching elements conductive simultaneously, no inter-power supply current will be generated in the power circuit. Thus, power consumption by the power circuit of the invention is greatly reduced.
The reference voltage circuit may include a resistor and a third switching element controlled by the voltage value at the output end and provided between the input end of the second comparator for receiving the reference voltage and either one of the first voltage supply and the second voltage supply.
In this arrangement, the reference voltage to the second comparator is automatically switched between two levels in accordance with the output of the second comparator. In other words, the arrangement adds to the second comparator a hysteresis character with respect to the output voltage.
The third switching element as well as the first and the second switching elements, respectively, can be MOSFETs.
Further, the first switching element can be an N channel MOSFET, the second switching element can be a P channel MOSFET, and the third switching element can be an N channel MOSFET.
The power circuit having this arrangement can control the switching elements involved at a very low power in response to the output voltages of the first and second comparators, respectively.
In accordance with another aspect of the invention, there is provided a display apparatus comprising a bias circuit, a buffer circuit electrically coupled to the bias circuit, a selection circuit electrically coupled to the buffer circuit, and a display panel electrically coupled to the selection circuit, wherein the buffer circuit is made up of the previously mentioned power circuit.